1. Field of the Invention
The present invention relates to a method and solution for forming or enhancing the properties, such as corrosion resistance, of a conversion coating on metal surfaces or substrates.
2. Background of the Related Art
In general, chemical conversion coatings are formed chemically by causing the surface of the metal to be xe2x80x9cconvertedxe2x80x9d into a tightly adherent coating, where either all or part of the conversion coating consists of an oxidized form of the substrate metal. Chemical conversion coatings can provide high corrosion resistance to the substrate as well as strong bonding affinity for paint. The industrial application of paint to metals generally requires the use of a chemical conversion coating, particularly when the performance demands are high.
Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly aluminum alloys with a high copper content, corrode much more rapidly than pure aluminum.
In general, there are two types of processes for treating aluminum to form a beneficial conversion coating. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the aluminum component and the chemical bath. The conversion coating formed on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
The second type of process is by chemically producing a conversion coating, which is commonly referred to as a chemical conversion coating, by subjecting the aluminum component to a chemical solution, such as a chromic acid solution, but without using an electric (current in the process. The chemical solution may be applied by immersion application, by manual application, or by spray application. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
Chromate based conversion coatings have been widely used in applications where maximum corrosion protection is an issue. Immersion of aluminum or aluminum alloys in a chromate conversion coating bath results in a thick, corrosion resistant film consisting of hydrated Cr(III) and Al(III) oxides. The reaction is driven by reduction of the high valent Cr(VI) ion and oxidation of the Al metal. Some of the benefits of a chromate conversion coating include hydrophobicity and self-healing properties.
Many aluminum structural parts, as well as Cd plated, Zn plated, Zn-Ni plated, and steel parts, throughout the aircraft and aerospace industry are currently being treated using this chromic acid process technology. Chromic acid conversion films, as formed on aluminum substrates, have been shown to meet a 168-hour corrosion resistance criterion, but they primarily serve as a surface substrate for paint adhesion. Because of their relative thinness and low coating weights (40-150 milligrams/ft2), chromic acid conversion coatings do not reduce the fatigue life of the aluminum structure.
However, environmental regulations in the United States, particularly in California, and in other countries are drastically reducing the levels of hexavalent chromium compounds permitted in effluents and emissions from metal finishing processes. Accordingly, chemical conversion coating processes employing hexavalent chromium compounds need to be replaced.
Some of the most investigated non-chromate conversion coatings used in the treatment of aluminum alloy-based materials are described as follows. Sol-Gel technology uses polymers or metal oxides either alone or mixed to form complexes by the hydrolysis of appropriate precursor compounds. Sol-Gels can form powders or thin films that inhibit corrosion on substrates.
Fluorozirconium coating technology uses complexed transition metal salts to create a thin film on a substrate material similar to a conversion coating. Specifically, zirconium is mixed with fluorine to create fluorozirconium, which reacts with the part surface to form a coating.
Cobalt-based coatings use cobalt and molybdenum to treat substrate materials. The coatings created are low in electrical resistance and are good for corrosion resistance.
Rare Earth Metal (REM) salts may be applied by heated immersion to create protective layers con substrate materials. REMs provide corrosion resistance by producing a protective oxide film.
Potassium permanganate solutions can be used to create manganese oxide films on substrates. Manganese oxide films resulting from potassium permanganate treatment closely match the corrosion resistance of traditional chromic oxide films used in conversion coatings. Potassium permanganate coatings can be very effective in protecting aluminum alloys.
Fluotitanic coatings, deposited from acid solutions with organic polymers, require few process steps, and can usually be done at ambient temperatures. Although these coatings have been widely used in a variety of applications, they have not been used in the aerospace industry.
Talc coatings, which are typically applied to aluminum substrates, are resistant to corrosion. These polycrystalline coatings are applied by precipitating aluminum-lithium compounds and other anions in an alkaline salt solution.
Anodizing is a process in which a metal surface is converted to an oxide layer, producing a tough, adherent surface layer. A thick oxide layer can be produced by immersing a part in an electrolytic solution and passing an electrical current through it, similar to electroplating. Then, by placing the part in boiling water, the film""s pores can be sealed. As a result, the oxide changes from one form to another.
Despite these alternatives, there is a continuing need for a conversion coating solution that will form a stable, corrosion-resistant conversion coating on metal surfaces without containing or producing toxic chemicals. Additionally, it would be desirable if the conversion coating provided a suitable surface for receiving organic coatings or paints.
The present invention provides a method for producing a conversion coating on a metal surface, such as aluminum, aluminum alloys, and ferrous metals. The method comprises forming a layer of boehmite on the metal surface, such as by exposing the metal surface to boiling water or anodizing the metal surface, and subjecting the layer of boehmite to an aqueous solution having an alkali metal molybdate selected from sodium molybdate, lithium molybdate, potassium molybdate, and combinations thereof to form the conversion coating. The boehmite layer is subjected to the aqueous alkali metal molybdate solution at a temperature between ambient and boiling, preferably between 60xc2x0 C. and 80xc2x0 C. and for a period of between 1 and 20 minutes.
The boehmite layer is subjected to the aqueous alkali metal molybdate solution by submersing, spraying, brushing or combinations thereof. The concentration of alkali metal molybdates in the aqueous solution is preferably less than 5% by weight, more preferably between 1% and 3% by weight, and even more preferably about 3% by weight. The aqueous solution may further comprise one or more alkaline metal passivators selected from lithium nitrate (LiNO3), sodium nitrate (NaNO3), ammonia nitrate (NH4NO3), and combinations thereof. Activators, such as lithium chloride or lithium fluoride, may also be included. Additionally, the solution may include an additive selected from potassium hexafluorozirconate (K2ZrF6), potassium hexafluorotitanate (K2TiF6), and combinations thereof. The resulting aqueous alkali metal molybdate solution preferably has a pH between about 7 and about 12.
Preferably, the conversion coating is treated with an aqueous calcium hydroxide solution prepared by combining calcium hydroxide with an aqueous solution having less than 1.73 grams of carbon dioxide per liter, preferably less than 0.85 grams per liter, and more preferably less than 0.45 grams per liter. The conversion coating is contacted with an aqueous calcium hydroxide solution for between about 1 and about 20 minutes, at a temperature between about 25 and about 100xc2x0 C. The aqueous calcium hydroxide solution has a calcium hydroxide concentration up to saturation, preferably between about 0.015 and about 0.15% by weight, and more preferably between about 0.06 and about 0.09% by weight.
The invention may further comprise subjecting the conversion coating to an aqueous post treatment solution having one or more compounds selected from alkali metal silicates, alkali metal borates, alkali metal phosphates, lithium nitrates, or combinations thereof, wherein the concentration of the one or more compounds is between about 0.1% and about 5% by weight. Preferably, the conversion coating is subjected to the aqueous post treatment solution at a solution temperature between about 10xc2x0 C. and about 100xc2x0 C., and for a period of between about 1 minute and about 20 minutes.